Tunable whip antenna with combined loading coil and shock spring



R. F. LEWIS TUNABLE WHIP ANTENNA WITH COMBINED LOADING May 3, 19663,249,945

COIL AND SHOCK SPRING 2 Sheets-Sheet 1 Filed July 5, 1962 INVENTOR.POBERTE LEW/s ATTORNEYS .and radiating element.

United States Patent Robert F. Lewis, Lincroft, N.J., assignor toProdelin,

Inc., Hightstown, N.J., a corporation of New Jersey Filed July 5, 1962,Ser. No. 207,746 Claims. (Cl. .343749) My invention relates to antennas,and more particularly, to a highly sensitive mobile whip antenna and toa novel mounting arrangement for such antenna on an automobile. I

In mobile radio communication, the system is designed to operate at apredetermined frequency within a narrow frequency band. Two-way mobilesystems generally use low power transmitters, and it is thereforedesirable to optimize the sensitivity of the whip antenna.

In regard to the antenna mounting arrangements, whip antennas haveutilized an insulator base usually porcelain to mechanically support theextending radiating element on the vehicle frame. The antenna iselectrically coupled to the vehicle transmitter/receiver by a shieldedlead-in cable in which the inner conductor is connected to the radiatingelement and'the shield is grounded to the vehicle frame.

When using the conventional type of whip antenna, foreign matter such asdirt, dust, water, films and the like tends to collect on the surface ofthe insulator base which over a period of time, reduces the antennaefiiciency. For example, the foreign matter causes stray low resistancepaths to be established over the surface of the insulator between theradiator and the vehicle frame and occasionally causing direct shorting.The foreign matter also accumulates electrostatic charges which causenoise to deteriorate further the performance of the antenna.

The whip antenna is conventionally resiliently mounted by a coil springattached between the base insulator When the radiating element hits anobstruction, the spring yields but the shock and zvibration aretransferred to the insulator. Further, when the vehicle is subjected tosevere vibration by riding over bumpy roads over an extended period oftime, the base insulator often suffers breakdown caused by cracking orchipping. When the .whip is used on military vehicles which often travelover rugged terrain and under adverse weather conditions, theaforementioned problems are severely aggravated.

Accordingly, it is an object of this invent-ion to provide a whipantenna, adaptable for mounting on a vehicle, and which eliminates theuse of an insulator base.

Another object of this invention is topr-c'vide a whip antenna mountedon a finite ground plane, through an electrical coil spring in seriestherewith, to provide an essentially resistive input impedance, andwhich resistance remains essentially constant over prolonged period ofusage.

A further object of this invention is to provide a coil spring mountingfor the whip antenna which acts as a series tuning coil; the coil beingprovided with an adjustable tap, the position of which determines theimpedance presented to the lead-in cable.

Still another object of this invention is to provide a whip antennahaving no base insulator and in which shocks and vibrations applied tothe whip are transmitted directly to the vehicle frame.

Still another object of this invention is to provide a whip antennahaving improved gain characteristics.

Yet another object oft-his invention is to provide a I "ice may be tunedto a preselected frequency without physically cutting the antenna.

Another object is to provide an efficient, reliable and rugged whipantenna.

Briefly, my invention comprises a whip element connected in series witha coil spring which is electrically connected to the vehicle frame. Thebottom of the coil spring is secured to a metal base or block which isattached to the vehicle frame. The metal base or block is provided withan opening through which the inner conductor of the lead-in cable maypass. The length of the antenna is selected to exhibit a capacitivereactance. The coil spring is provided with a movable tap, electricallyconnected to the'lead-in, to cancel the capacitive reactance and toprovide a matched impedance to the antenna.

The above-mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent and the inventionitself will be best understood by reference to the following descriptionof an embodiment of the invention taken in conjunction with theaccompanying drawing, wherein:

FIGURE 1 is a partially exploded and a partial vertical sectional viewthrough the center of the whip antenna showing the internal constructionthereof;

FIGURE 2 is a sectional view showing the coaxial cable prepared forassembly; and

FIGURES 3 and 4 are various plots of VSWR-frequency for differentmounting positions.

Referring now to FIGURE 1, there is shown a whip antena mounted on avehicle 2 or finite ground plane. The radiating element '1 is firmlyconnected to the upper end of a coil spring 3 which provides thenecessary fiexibility in mounting.

In accordance with a first aspect of the invention, a conductive basemount block 6 is used instead of the insulative base. threaded andscrewed into a tapped hole in the block and locked thereto by a lock nut5. The metal block is a strong and rugged member which supports therelatively large whip and transfers shocks directly to the vehicleframe. As will be seen, the metal block is in direct electrical andmechanical contact with the vehicle. frame 2. Connected to the base ofthe metal block is a conductive extension or stud 7 having an externalthread 8 at its lower end and having an internal opening 9. A similaropening 10 extends centrally through the metal block. The vehicle frameis provided with an opening to receive the threaded extension 8 of thestud 7.

A tuning clip 11, shaped for slidable movement, substantially encirclesa turn of the coil spring and is clamped to a turn at a selected pointby tightening the bolt and nut 12. The clip may be easily moved byloosening the nut and applying asliding force. A flexible wire 14connects the tuning clip to a terminal 13 insulatively mounted on thetop surface of the base 6.

The coil spring is provided with a sufficient number of turns to as tocancel the capacitive reactance of the antenna, as will be describedshortly.

A circumferential recess or groove 15 is formed in the outer surface ofthe base 6 and a boot 16 having corrugated or bellow-like sidessurrounds the entire coil spring and the top surface of the conductfngblock. When the antenna is subjected to shocks or vibrations, the bootand spring are shaped to flex, bend or twist. It is apparent that theantenna mounted on an automobile will be in a constant state of lateralmovement relative to the base, thereby causing the boot to flex andinhibit the formation of a continuous stray path such as one of ice orsnow.

The boot is a cup-shaped member preferably formed of an elastomericmaterial such as rubber, and preferably a low loss rubber but itsresilience is achieve-d by the cor- The other end ofthe coil springisrugated side Walls 17. It has a central aperture 41 in the top surfacethrough which the whip passes. The lower side wall has an inwardlyextending peripheral bead 40 which is snap fitted below the shoulder ofthe recess 15. The antenna fits tightly in the aperture 41 so as toprevent foreign matter from entering the interior of the boot and causeundesired stray impedance paths.

The whip antenna is mounted on the vehicle frame in conventionalfashion. Preferably, tapered washers 2t) and 21 are mounted on oppositesides of the vehicle frame. There washers may be adjusted to positionthe antenna vertically on the vehicle. A rubber gasket 22 is tightfitted on the stud 7 and is positioned against the vehicle to serve as aseal. Lockwashers 23 and 24 employed in the conventional manner and jamnut 25 clamps the whip to the vehicle frame.

The cable is prepared conventionally by stripping off sections of thejacket, braiding and insulator respectively leaving parts of eachexposed. The exposed braiding 3t) fits into the hollow of a clamp nut 26(FEGURES 1 and 2). The tapered end of a clamp collar 27 also embracesthe braiding as the nut 26 is tightened on the collar. The innerconductor 32 is slipped through insulator 28 which protects it fromcontacting the conductive block 6 and extension 7. The inner conductor32 makes direct contact with the conductive element of terminal 13. Theinsulator along with the entire cable assembly is assembled into thewhip. Locking nut 35 secures the entire assembly.

The novel whip antenna operates preferably in the frequency range of144l64 or 144174 megacycles/second although by means of the seriestuning coil higher selectivity and performance are obtained at narrowfrequency bands within the specified range. It is possibie to obtain aVSWR (voltage standing Wave ratio) remarkably close to 1 by tuning outthe capacitive reactive component of the antenna and carefully selectinga feed point on the series coil to match the impedance of the lead-incable.

My antenna is cut to a length such that the base impedance is preferablycapacitive and I have found that a rod length of A; waveiength(approximately 59" maximum) is satisfactory. I obtain considerablevariation in VSWR by adjusting the tap point on the coil as shown inFIGURE 3.

FIGURE 3 shows the VSWR over a range of frequencies for preselected tappoints of 0; 1; 1.5; 2.0; 2.5; 3.0; 3.25 and 3.50 turns. Of course, tappoints intermediate the selected tap points yield better results forcertain frequencies. Thus, for a frequency of approximately 151.5 me. atap point at the end of the second turn would provide a substantiallymatching impedance with a 50 ohm lead-in cable.

The vehicle frame upon which the whip antenna is mounted acts as afinite ground plane. erably mounted centrally on the vehicle frame withrespect to surrounding metal to yield better impedance characteristicsand pattern configuration than one which is edge mounted on a vehicle.FIGURES 3 and 4 illustrate the results achieved with two different whipmountings. In FIGURE 3, the whip was mounted centrally while in FIGURE4, the whip was mounted on the fender. It is seen that the VSWRincreased over the upper range of frequencies in FIGURE 4. it can alsobe shown that the radiation patterns in the horizontal and verticalplanes are more distorted when the Whip antenna is not centrallymounted.

The presence of a finite ground plane makes it ex- The Whip is preftilCir

tremely difiicult to calculate the input impedance of a It is seen thatthe invention provides a simple means for tuning out the capacitivereactance to match the input impedance to that of the coaxial lead-inthereby producing improved standing wave ratios. For proper tuning it isonly necessary to slide the boot up the whip enough to expose the coiland tuning clip and adjust the clip for peak radiation. Thus physicalcutting of the whip in the field to obtain proper match to thetransmission line is no longer required. When properly tuned, my antennapossess approximately a 2.5 db gain over a A Wavelength antenna in thehorizontal plane when mounted to the same vehicle and in the same placeWhile also eliminating the base insulators which frequently developshock failures. The series tuning coil provides mechanical flexibilityand is exceptionally rugged because all stresses are transferred to thevehicle via metal and not an insulator.

While the foregoing description sets forth the principles of theinvention in connection with specific apparatus, it is to be understoodthat the description is made only by way of example and not as alimitation of the scope of the invention as set forth in the objectsthereof and in the accompanying claims.

I claim:

1. A whip antenna for use on a vehicle frame comprising an extendingradiating element,

rigid conductive means mounted on said frame in electrical contacttherewith, said frame forming a finite ground plane for said antenna,

means for securely connecting said rigid conductive means to said frameso as tobe in both mechanical and electrical contact therewith,

said conductive means having an aperture extending therethrough toreceive a lead-in conductor,

resilient helical means having a plurality of turns of a relativelystiif wire to resiliently support said radiating element and having apredetermined inductance,

one end of said helical means being rigidly attached to said radiatingelement andthe other end being rigidly and conductively secured to saidconductive means,

means to couple said helical means to said lead-in conductor comprisingmovable conductive tap means coupled to said helical means,

and a protective boot surrounding said helical means to prevent theformation of stray electrical paths by foreign matter on said helicalmeans and on said lead-in conductor,

' said tap means being movable along the turns of said helical means andproviding a secure electrical connection thereto, whereby saidconductive means transfers shock and vibrational forces to the vehicleframe and said coil serves the dual function of a resilient spring mountand an electrical inductance.

2. A whip antenna for use on a vehicle frame comprising an extendingradiating element,

a conductive base mounted on said frame in substantially rigid andconductive relationship, said frame forming a finite ground plane forsaid antenna,

said conductive base having an aperture extending therethrough toreceive a lead-in conductor,

resilient coil means having a plurality of turns of a relatively stiffwire to resiliently support said radiating element and having apredetermined inductance,

one end of said coil means being rigidly attached to said radiatingelement and the other end being rigidly and conductively secured to saidconductive base,

means to couple said coil means to said lead-in conductor comprisingmovable conductive tap means coupled to said coil means,

said tap means being movable along the turns of said coil means andproviding a secure electrical connection thereto,

said radiating element having a length so that the reactance at the basethereof is capacitive,

said coil means having an inductance value to compensate for saidcapacitive reactance,

a protective covering surrounding said coil means,

and a lead-in cable having a predetermined impedance,

said tap being positioned to obtain matching with said impedance,whereby said base transfers shock and vibrational forces to the vehicleframe and said coil serves the dual function of a resilient spring mountand an electrical inductance.

3. A whip antenna for use on a vehicle frame comprising an extendingradiating element,

a conductive base adapted to be mounted to said frame in electricalcontact therewith, said frame forming a finite ground plane for saidantenna,

said conductive base having an aperture extending therethrough toreceive a lead-in conductor,

resilient coil means having a plurality of turns of a relatively stiffwire to resiliently support said radiating element and having apredetermined inductance,

one end of said coil means being connected to said radiating element andthe other end being rigidly and conductively secured to said conductivebase,

means to couple said coil means to said lead-in conductor comprisingmovable conductive tap means coupled to said coil means,

said tap means being movable along the turns of said coil means toprovide a variable inductive reactance,

means to secure the one wire of said lead-in conductor conductively tosaid base and to secure the other wire conductively to said tap means,

and a resilient protective cover surrounding said coil means andextending from said base to said radiating element,

the range of said inductive reactances being such as to compensate forcapacitive reactance presented by said radiating element, whereby saidbase transfers shock and vibrational forces to the vehicle frame andsaid coil serves the dual function of a resilient spring mount and anelectrical inductance.

4. A vehicular antenna system comprising a conductive vehicle frame, anextending radiating element,

a conductive base mounted directly on said frame, said frame forming afinite ground plane for said radiating element,

said conductive base having an aperture extending therethrough toreceive a lead-in conductor,

resilient helical coil means having a plurality of turns of a relativelystiff wire to resiliently support said radiating element and having apredetermined inductance connected in series with said radiatingelement,

one end of said helical means being rigidly attached to said radiatingelement and the other end being rigidly and conductively secured to saidconductive base, a protective resilient boot surrounding said helicalmeans to prevent the formation of stray electrical paths by foreignmatter on said helical means and on said lead-in conductor,

and means to couple said helical means to said lead-in conductorcomprising a movable conductive tap means coupling selected turns tosaid lead-in conductor to match said lead-in conductor, whereby saidbase transfers shock and vibrational forces to the vehicle frame andsaid coil serves the dual function of a resilient spring mount and anelectrical inductance.

5. A whip antenna for use on a metal vehicle frame comprising aradiating element,

a conductive base adapted to be mounted on said frame in electricalcontact therewith, said frame forming a finite ground plane for saidantenna,

a lead-in conductor extending through an opening in said base,

a coil having a predetermined number of turns of relatively stiff wire,

one end of said coil being connected to said radiating element and theother end being conductively secured to said base and a movable tapcoupling selected turns of said coil to said lead-in conductor, wherebysaid base transfers shock and vibrational forces to the vehicle frameand said coil serves the dual function of a resilient spring mount andan electric inductance,

and a .slidable protective boot means covering said helical means andbeing anchored to said base, the sides of said boot being flexible,whereby said sides may follow the expansion of said helical means.

References Cited by the Examiner UNITED STATES PATENTS 2,498,350 2/1960Walsh 343-900 HERMAN KARL SAALBA'CH, Primary Examiner.

2. A WHIP ANTENNA FOR USE ON A VEHICLE FRAME COMPRISING AN EXTENDINGRADIATING ELEMENT, A CONDUCTIVE BASE MOUNTED ON SAID FRAME INSUBSTANTIALLY RIGID AND CONDUCTIVE RELATIONSHIP, SAID FRAME FORMING AFINITE GROUND PLANE FOR SAID ANTENNA, SAID CONDUCTIVE BASE HAVING ANAPERTURE EXTENDING THERETHROUGH TO RECEIVE A LEAD-IN CONDUCTOR,RESILIENT COIL MEANS HAVING A PLURALITY OF TURNS OF A RELATIVELY STIFFWIRE TO RESILIENTLY SUPPORT SAID RADIATING ELEMENT AND HAVING APREDETERMINED INDUCTANCE, ONE END OF SAID COIL MEANS BEING RIGIDLYATTACHED TO SAID RADIATING ELEMENT AND THE OTHER END BEING RIGIDLY ANDCONDUCTIVELY SECURED TO SAID CONDUCTIVE BASE, MEANS TO COUPLE SAID COILMEANS TO SAID LEAD-IN CONDUCTOR COMPRISING MOVABLE CONDUCTIVE TAP MEANSCOUPLED TO SAID COIL MEANS,